Optical signal processing systems are used to manipulate the characteristics of optical signals, or light beams. For example, the direction, intensity, polarization, phase (or a combination of such characteristics) of a light beam may be manipulated by appropriate equipment so that the manipulated characteristics represent the desired processing of the optical signal.
An essential component in most optical processing systems is an efficient light switch. Key characteristics for a light switch used in a signal processing system include the time for an optical switch to be changed from one position to another and the amount of loss the optical signal experiences in the switch (particularly in systems in which the processed light must pass through many switches). For example, a commonly used light switch is a lithium niobate integrated optic type of switch, which, although it has relatively fast switching times (e.g., about 5 nsec.), typically has about 3 dB light loss per switch. Thus, for example, if an optical signal passed through only seven switches in a signal processing system, such as a 7-bit binary optical delay line, its intensity would drop 21 dB, that is, the light amplitude of the output signal drops to 1/128th of the input amplitude.
A type of light switch that offers efficient, low light loss operation includes a beam splitter and a liquid crystal array to selectively control the polarization of light beams entering the beamsplitter. Nematic liquid crystal (NLC) switching arrays typically are relatively slow, at least with respect to the switching mode in which the control voltage is reduced across the liquid crystal and the liquid crystal molecules "relax", that is, the orientation of the molecules changes to correspond to the reduced electric field. For efficient polarization-based switching, the polarization orientation of the light passing through the liquid crystal molecules must be rotated by 90.degree.; the time for the liquid crystal molecules to be displaced sufficiently to cause such a .pi.-radian phase shift of the light can range from 30 msec to 5 msec, dependent on cell characteristics such as thickness, NLC mixture birefringence, the optical wavelength, and the applied voltage.
Several different operating regimes have been suggested to improve NLC switching times. For example, "surface mode" operation (also referred to as the bias voltage effect), discussed by J. L. Fergason in "Performance of a Matdx Display Using a Surface Mode", IEEE 1980 Biennial Display Research Conference Proceedings, pp 177-179 (incorporated herein by reference), involves using changes in the orientation of the molecules near to the sides of the cell to effect a polarization change in light passing therethrough. Surface mode operation involves applying a relatively high bias voltage across the NLC such that substantially all the liquid crystals except those close to the sides are aligned with the high electric field; switching is accomplished, for example, by reducing the electric field to a lower value, thereby causing relaxation of liquid crystal molecules near the sides of the cell. This type of operation improves switching speeds, although the switching times dependent on relaxation of the molecules is still relatively long (e.g., 5 msec.).
Another approach to improving NLC switching times, referred to as the "transient nematic liquid crystal (LC) effect" is described in "Small angle relaxation of highly deformed nematic liquid crystals," by S-T. Wu and C-S Wu in Applied Physics Letters, volume 53, pp 1794-1796 (November 1988), and incorporated herein by reference. The transient nematic effect also involves maintaining a relatively high bias voltage across the LC cell (such that substantially all LC molecules except in the barrier layer are aligned with the field, similar to the surface mode operation described above). In the switching mode in which the molecules relax, the bias voltage is removed entirely, allowing the directors to undergo free relaxation until a desired (minimum) transmitted light intensity is reached, at which a second control voltage is applied (lower than the original control voltage) to stop the motion of the LC directors. This method provides some improvement in switching speed, but remains limited to speeds of about 1 msec. The transient nematic LC effect thus generally provides about a fivefold improvement in operating time (that is, faster switching time) dependent on conditions such as LC temperature, birefringence, and cell thickness.
It is desirable that switching units in optical signal systems have relatively fast speed operation (e.g., a switching time of less than about 100 .mu.sec.), exhibit low attenuation, be of compact size, rugged, readily fabricated, and adapted to processing a large number of separate signal light beams (e.g., up to thousands of beams) as would be necessary for operation of a phased array transducer system. It is further desirable, from the standpoint of manufacturing ease and efficiency, that each optical switch comprise as few optical devices, such as beamsplitters, as practicable.
It is accordingly an object of this invention to provide an optical switching unit having relatively fast speed switching and that is readily adapted to use in a cascade of other optical processing devices.
It is a further object of this invention to provide a fast speed optical switching unit that is readily fabricated in a cascade of similar devices.